Transgenic mouse models of human cancers present unique opportunities to elucidate important cellular and genetic pathways of cancer development, through which normal cells progressively are converted into aberrant cancers. By studying a transgenic mouse model of squamous cell carcinoma development, we have identified an important paracrine modifier of epithelial carcinogenesis, e.g., gelatinase B/matrix metalloproteinase-9 (MMP9), whose role previously was believed to facilitate basement membrane degradation. During neoplastic development, MMP-9 regulates epithelial cell proliferation, differentiation, and overall malignancy of emergent cancers, all previously unappreciated regulatory capabilities for this extracellular proteinase. These realizations imply that MMP-9 exerts a more profound role during early events in tumor evolution. The overall goal of this proposal is to identify the target epithelial cells regulated by MMP-9, identify molecules that interact with MMP9 mediating proliferative responses and, identify intracellular signal transduction pathways activated as a consequence of MMP-9-induced proliferation. We will address the characteristics and functional significance of these parameters/mechanisms by combined in vivo and in vitro approaches. The specific aims of this project are: Aim 1. To determine mechanisms regulating neoplastic epithelkil cell proliferation by MMP-9. We will determine the identity of the target epithelial cells responsive to MMP-9 and assess candidate protein substrates of MMP-9 mediating proliferative responses. Aim 2. To determine the mechanism(s) of activation responsible for MMP-9 -induced proliferation. Using in vitro cell culture systems that phenocopy proliferative responses to MMP-9 observed in vivo, we will determine the identify of critical protein targets and intracellular signal transduction pathways specifically activated by MMP-9. Aim 3. Determine the impact of MMP-9 on the initiation, promotion, and malignant conversion steps of multistage epithelial carcinogenesis. The functional importance of MMP-9 at discrete steps of multi-stage epithelial carcinogenesis will be ascertained using the classical 'two-stage' model of chemical carcinogenesis. These experiments will reveal if MMP-9 plays a more significant modifier role at the initiation, promotion, or malignant conversion stages of cancer development. The immediate ramifications of this work are in its applications to use of MMP-Inhibitors (MMPIs) as anticancer agents. Only a thorough understanding of the actions and effects of MMP-9 will effectively guide use of MMPIs in the treatment of cancer.